Glaciers continue to shrink, ice sheets at risk in the long term

We're getting a better sense of the changes that will occur if climate change …

Over the weekend, Nature Geoscience released two papers that provide some perspective on the changes that are likely to take place as our planet continues to warm. One took advantage of an expanded catalog of glacier measurements to track how they would respond to the changes by the end of this century, and shows continued losses, with a peak rate in the 2040s. The second takes a longer-term view, tracking the effects of our current emission trajectory out to the year 3000. That study suggests that, even were we to cease all carbon emissions by 2100, it will take the planet hundreds of years to reach a new equilibrium, and that the process of doing so could drive some radical changes.

First, the glaciers. In response to the rapid shrinking of glaciers, researchers around the world have upped their monitoring of ice loss, leading to data set that tracks changes in 120,000 glaciers. The authors created an elevation-dependent mass-balance model of glacial response to temperatures, and then calibrated it using historic data from these glaciers. They then ran the model forward to 2100, using temperatures derived from an ensemble of 10 different climate models, using a mid-range scenario for the trajectory of future carbon emissions.

Obviously, the models differed in their responses to the increased greenhouse gasses, which led to a range of results as far as the glaciers were concerned. Still, the unmistakable trend was downwards, and the results provide the finest resolution map of the changes we have to date.

The typical loss was about 20 percent, but differences varied widely. The hardest-hit areas are projected to be Europe and New Zealand, where losses approached 70 percent. But because the total ice there is relatively small, this contributed little to sea level changes; instead, those were dominated by Canada, Alaska, and Antarctica. Excluding the large ice sheets in Greenland and Antarctica, glacial melt was projected to contribute about a millimeter a year to ocean level changes. Given the losses that have already occurred and a projected decrease in carbon dioxide emissions, the rate of loss is actually expected to peak fairly soon, sometime in the 2040s.

Overall, the study projects a sea level rise of about a dozen centimeters due to the loss of ice from these glaciers, although the authors indicate this is almost certainly an underestimate, since it doesn't include the calving of icebergs from those glaciers that terminate in the ocean. Still, it's the most detailed regional projection yet, and can give those countries that rely on glacial melt for water a better sense of what to plan for.

If this study of glaciers was a very cautious one—it used 10 different models, and a middle-of-the-road projection for carbon dioxide emissions—then the long-term study was more of a "let's see what happens" effort. The work used a single model, the Canadian Earth System Model, and a rather extreme emissions projection. They take one of the IPCC's high-emissions scenarios, but cut all emissions off entirely in 2100. Needless to say, this isn't realistic, but it would give some sense of what loading up the atmosphere with greenhouse gasses now might do in the future.

By the year 3000, over half of the CO2 is still in the atmosphere. There's a rapid decline driven by the terrestrial biosphere once emissions stop, but that ends within a century. Once emissions stop, temperatures largely stabilize after an additional increase that, in many locations, is equivalent to the warming that's occurred since 1825. They stay put because delayed temperature increases are largely balanced by the decreasing atmospheric concentrations.

But the model suggests that, although the global temperature stays relatively stable, there are significant regional changes. The Northern Hemisphere actually cools down slightly by 3000, as do most of the tropics. But the coast of Antarctica continues to rise, adding an additional 9°C after 2100. The other big change comes in the ocean, as areas of significant turnover remain warm for centuries. This gradually heats the middle depths of the ocean, which transports some of the heat to Antarctica. The elevated ocean temperatures may also destabilize any glaciers that terminate in the ocean, much as we see currently happening in Greenland. The warming of the oceans is also sufficient to drive a meter of sea level rise.

The net result is that, centuries after CO2 emissions stop entirely, the West Antarctic Ice sheet, which holds enough water to raise ocean levels by several meters, may become unstable.

Clearly, with a single model and a rather unrealistic emissions scenario, this can't be considered a likely projection of future climate events. However, it does highlight the fact that we can't expect the planet to adopt an equilibrium that looks a lot like today, but warmer. And the authors note that some have argued that we can turn to geoengineering if it ever looks like a major ice sheet is starting to become unstable. As their scenario points out, by the time we notice a problem, we might have already been committed to an ice sheet collapse centuries earlier.